(a) Field of the Invention
The present invention relates to an ultra-wide angle f-.theta. lens system and, more particularly, to an ultra-wide angle f-.theta. lens system with a scanning angle (field angle) widened to .+-.45.degree..
(b) Description of the Prior Art
As office automation systems are coming into wider use in these days, it is required to provide a more compact laser beam printer at a still lower price. To make an office automation system more compact, it is necessary to arrange that the f-.theta. lens system to be used therein has an ultra-wide scanning angle.
However, in case of standard type f-.theta. lens systems which are already put on the market, the scanning angle thereof is not wider than .+-.25.degree.. Even in the case of wide angle f-.theta. lens systems, the scanning angle thereof is not wider than .+-.30.degree.. In other words, there exists almost no known f-.theta. lens system which has a scanning angle wider than .+-.30.degree. and, at the same time, which shows an imaging performance of laser beam waist close to the diffraction limit on the light-sensitive drum surface in the state that the scanning linearity (f-.theta. characteristics) is ensured to the degree that is satisfactory for practical use (.+-.0.7% or smaller).